Im creating several LCD meters for a friend of mine to monitor voltages/current on his rack power equipment(telecom). It operates at 48v negative GND. So all is well and good up to this point, LCD meter working aok. But his equipment can be switched to handle 48v positive GND. With this scenario I could use a bridge diode to the supply of the uC to ensure that the + and - are always correct. My problem is the ADC should be able measure voltages on either GND orientations. Meaning some are +gnd and some are -gnd, so having an isolated power supply wont help. I've looked at the Ti INA117 differential opamp and use it as Differential to single ended ADC input buffer but dont know exactly how to use it (not to mention pricey).

So I think I have narrowed the solution(with my limited electronics knowledge)to having a differential input buffer to the ADC to isolate the monitored voltages but just dont know how to implement it.

Well a partial answer to your application might be to use a diode bridge rectifier module (or 4 didodes wired as a bridge) wired to any DC power source you want to monitor, to the AC input terminals of the bridge. That way the output terminals will always be the same polarity regardless of the input polarity. There is a two diode voltage drop to factor for, but otherwise a neat little trick.

I am making the meter, its an arduino with LCD. It monitors current, voltages, battery temp and a low voltage disconnect. Yes, these are pretty major stuff but the meters are only for monitoring. I think the problem is with the ADC measuring multiple voltages at different GND levels. So its more of a data acquisition type of equipment than a regular digital meter. If it only measures 1 voltage then an isolated power supply will work just fine.

retrolefty,

Thats what Im using on the supply line to the arduino. I also tried placing a bridge diode on the 48v then to the divider resistor, its linearity is way off. So I dont think that will work well.